Oxidation of thin WSi2 overlayers on Si(111)

Abstract

The mechanisms and kinetics of oxidation of thin WSi2(0001) epitaxial layers (∼150 Å) prepared by annealing W deposits on Si(111)(7×7) surfaces have been studied by ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), and x-ray induced Auger electron spectroscopy. Oxidation was carried out at room temperature and at 700 °C, under 2×10−5 mbar oxygen pressure. A Si(111) sample, submitted to the same oxidation procedure, was used as a reference. The energy positions and the intensity variations of the core levels (Si 2p, W 4f) and XPS valence bands indicate for both oxidation conditions (low and high temperature) the formation of silicon oxide on top of the WSi2 layer, while the composition and the electronic structure of this silicide are unaffected. Oxygen is bonded to Si only and no W–O bond is detected. The nature of the silicon oxide overlayer depends on the temperature. At room temperature, the oxide derived Si 2p feature corresponds to a dominant O–Si–O bonding, to be compared to the multiple configuration observed with the Si(111) face. Additional experiments with one monolayer of W deposited on Si(111) show an enhancement of oxidation and migration of Si atoms from the bulk Si to the surface. The UPS spectra and the work function measurements agree with an atomic chemisorption of oxygen. At 700 °C, silicon dioxide grows on the surface. In this case, the Si KL2L3 Auger transition, owing to its larger energy shift and a larger escape depth in comparison with the Si 2p, reveals the presence of a 3–5 Å suboxide region between the SiO2 layer and the underneath WSi2. The oxidation rates are comparable to those of Si(111).